Most of the modern communication systems generally use frequency modulation techniques to transmit analog and digital information between a transmitter and a receiver. At present the sate of the art in digital data transmission over narrow bandwidth is by using Frequency Modulation (FM) and Frequency Shift Keying (FSK). A detailed description of this technique and its background art and specifications is described in the following paragraphs.
U.S. Pat. No. 5,852,636 for Mathieu et al., describes Methods and apparatus for modulation of a Frequency Shift Keying (FSK) carrier in a very narrow band. This patent provides a method and a transmitter system for modulating binary information on a carrier in a very narrow band by Frequency Shift Keying (FSK) a carrier signal. Instead of the direct use of the frequency shift keying technique, a phase reversal keying technique is used to create a carrier with frequency shift characteristics.
The transmitter of the above referred patent generates a reference signal and a control signal, the latter having two frequencies to represent the binary data to be transmitted. A carrier signal is generated by modulation using a phase reversal technique. The resulting carrier signal is processed by high order filtering such that the output of the transmitter provides a Frequency Shift Keyed (FSK) carrier at very close frequencies. However, Mathieu fails to disclose a system and method for transmitting and receiving digital signals using Frequency Shift Keying (FSK), Frequency Modulation (FM) and using multiple binary orthogonal frequencies.
U.S. Pat. No. 5,358,202 for Tse et al. discloses a Frequency Shift Keying (FSK) demodulator using a phase locked loop and voltage comparator. The U.S. Pat. No. 4,456,985 of R. T. Carsten et al., discloses an arrangement in which full duplex above voice band Frequency Shift Keyed (FSK) data may be transmitted simultaneously with telephone signals over a subscriber's telephone signals which may be present, and they must be sufficiently high to distinguish them from voice band signals and sufficiently low that they are not unduly attenuated by the telephone line. For example, the Frequency Shift Keyed (FSK) center frequency being changed by +0.500 Hz depending on whether a data 0 or 1 is being transmitted.
In consequence, the Frequency Shift Keying (FSK) center frequencies are relatively inaccurately determined and may be subject to change due to the combined effects of component tolerances aging, and temperature changes.
With conventional Frequency Shift Keying (FSK) demodulation, the effect of an incorrect center frequency in the incoming Frequency Shift Keying (FSK) data is to produce a distorted mark/space ratio in the demodulated data. In the arrangement discussed above the distortion can be severe because the change in the Frequency Shift Keying (FSK) center frequency can be significant compared with narrow band (+0.500 Hz) which is used for Frequency Shift Keying (FSK) transmission. Such distortion can lead to errors in examining the demodulated data if transitions of the demodulated data are used to determine the times at which this data is sampled.
U.S. Pat. No. 4,486,955 to Mass et al., discloses a Frequency Shift Keying (FSK) demodulator. This demodulator is described as a circuit for detecting differences in frequency between a Frequency Shift Keying (FSK) modulated input signal and a reference signal (REF). The circuit includes a sequence generator means for producing a digital code representative of phase angle between the Frequency Shift Keying (FSK) and reference signal (REF) signals. The digital code (which is preferably a two bit binary code) changes a first predetermined sequence when the frequency of the Frequency Shift Keying (FSK) signal is less than the frequency of the reference signal (REF), and changes in a second predetermined sequence when the frequency of the Frequency Shift Keying (FSK) signal is greater than the frequency of the reference signal (REF).
The circuit, as disclosed by Mass et al., includes a first sequence detector means for detecting the occurrence of the first predetermined sequence, and a second sequence detector means for detecting the occurrence of the second predetermined sequence. Each time one of the sequence detector means provides an output indicating that its selected sequence has been detected; it is reset to begin another sequence detection cycle. The outputs from the second sequence detector means during a predetermined period are loaded into first and second integrating shift register means. At the end of the predetermined period, decision means compares the contents of the first and second integrating shift register means, and provides a data output based upon the contents of the first and second integrating shift register means. In this way, the sequence which was detected the most times during the period determines the data output from the decision means. Also, the above patent to Mass et al., discloses a method for estimating the frequency of a time signal by means of a discrete Fourier transformation and interpolation, without analyzing sampled data using over sampling. It also uses zero padding in a multi sample, multi frequency message with high speed to determine the data content of the message.
Advantageously, the present invention is applicable for both one carrier Narrow Band Frequency Modulation (NBFM) system and for a multiple carrier system. For multiple carrier transmission systems (each carrier has a mark and a space frequency). Either a Band Pass Filter (BPF) or matched filter schemes are used to separate the frequencies in a multiple carrier parallel system.
Further, the present invention provides a Frequency Shift Keying (FSK) demodulator that can be used to demodulate Narrow Band Frequency Shift Keying (NBFSK) data. The Frequency Shift Keying (FSK) provides reduced distortion of the mark/space ratio of the demodulated data in the event of a variable Frequency Shift Keying (FSK) center frequency where a phase locked loop is used to determine frequencies. The demodulator preferably includes a low pass filter for coupling the output voltage of the phase locked loop to the D.C blocking means.